igneous
Polished black stone, likely obsidian
Volcanic glass; primarily silica-rich amorphous SiO₂
AI-generated identification · may be incorrectTypically black with vitreous luster, no visible crystals or cleavage, conchoidal fracture, and Mohs hardness about 5–5.5; this specimen is tumbled and polished. The image alone cannot exclude black agate, onyx, or another dark stone.
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Physical properties
Typically black with vitreous luster, no visible crystals or cleavage, conchoidal fracture, and Mohs hardness about 5–5.5; this specimen is tumbled and polished. The image alone cannot exclude black agate, onyx, or another dark stone.
Formation & geological history
Obsidian forms when silica-rich lava cools rapidly, preventing crystal growth, usually in Quaternary to Miocene volcanic settings. Common sources include Mexico, the western United States, Armenia, and Japan.
Uses & applications
Used for ornamental objects, cabochons, beads, carvings, and historically sharp tools and arrowheads; it has limited construction use.
Geological facts
Its glassy texture reflects rapid volcanic cooling rather than ordinary crystal growth. Snowflake, mahogany, rainbow, and sheen varieties contain inclusions or flow structures.
Field identification & locations
Identify by glassy appearance, sharp conchoidal edges, and lack of grain; a streak test is usually white, while a steel tool may scratch it. If strongly banded or waxier, it may instead be black agate or onyx.
